This invention relates to an optical transmission line whose properties remain unchanged under the application of external forces, especially compressive forces, and a flat cable comprising a plurality of such transmission lines.
It is common practice to lay circular communication cables along floor surfaces for indoor telephone wiring, or for wiring between electronic machines. Recently, however, with developments in office automation systems and information processing machines, wiring has been reviewed with greater interest, and it has been proposed to lay flat cables beneath carpet tiles, for reasons of visual appearance, ease of laying, and safety. For this purpose, an extremely thin, flat cable, a so-called undercarpet cable, has been developed. Further, in the communication systems for office automation, especially for the communications between electronic computers and their terminals, optical transmission lines have been devised and employed to accomodate increases in channel capacity. Utilization of optical transmission lines in undercarpet cables is also desired.
However, as is well known, an optical transmission line is vulnerable to external forces, so that when such a line is used in a situation where it is likely to be subjected to large external forces, such as when it is used in an undercarpet cable, specific protection and reinforcement of the line is required. For this purpose, a protective layer and a reinforcing line or lines are usually provided such as those disclosed in U.S. Pat. Nos. 4,185,887; 4,00,932 and 4,231,635. An optical transmission line 1 illustrated in FIG. 1 has been provided. The optical transmission line 1 has a structure in which a buffer layer 3 of silicone resin is provided around an optical light guide or fiber 2 of quartz, glass or certain plastics, and a plurality of reinforcing strands 4 are placed lengthwise along and surrounding the buffer layer 3, the reinforcing strands 4 being held in position by tape 5 wound therearound, and a sheath 6 made of PVC or a similar material is provided covering all the component parts. The optical transmission line 1 can be used singly, but it is also possible to connect a plurality of these lines laterally to form a flat cable 7, as shown schematically in FIG. 2.
However, because this known optical transmission line has a circular cross-section, it tends to twist or distort when it is laid along a floor surface. Such a strip also has insufficient resistance to compressive forces, and deformation of the sheath 6 is bound to affect the optical transmission line 2, so that when it is used where large external forces are applied, such as when it is laid below carpet tiles, the transmission characteristics of the line can be greatly impaired. The connection of these lines to terminal strips, connectors, etc., is difficult and when a plurality of these lines are combined and used in a flat cable, as shown in FIG. 2, unsatisfactory resistance to compressive forces results.
The present invention has been developed in view of defects in prior art transmission lines. An object of the invention is the provision of an optical transmission line with improved resistance to external forces, especially compressive forces, which can be used for an undercarpet cable or a flat cable generally.